The strikes were help from the edge of the Arctic Circle to the South Pacific.

Weekly protests have snowballed from a handful of cities to hundreds, driven by social media-savvy students and dramatic headlines about the impact of climate change.

Greta, who was recently nominated for the Nobel Peace Prize, was cheered for her blunt message to leaders at the World Economic Forum in Switzerland this year, when she told them: "I want you to panic. I want you to feel the fear I feel every day."

Thousands of Dublin students march from St Stephens green to Leinster House (Image: Niall Carson/PA Wire)

Friday's rallies are expected to be one of the biggest international actions yet. Protests were being held in cities in more than 100 countries including Hong Kong; New Delhi, India; Wellington, New Zealand; and Oulo, Finland.

In Berlin 10,000 protesters, most of them young students, gathered in a central square waving signs with slogans such as "There is no planet B" and "Climate Protection Report Card: F", before a march through the capital's government quarter.

In Poland, thousands marched in rainy Warsaw and other cities to demand a ban on the burning of coal. Some wore face masks as they carried banners that read "Today's Air Smells Like the Planet's Last Days" and "Make Love Not CO2".

In India's capital New Delhi, schoolchildren protested over inaction on climate change and rising air pollution levels that often far exceed World Health Organisation limits.

About 50 students protested in South Africa's capital Pretoria, chanting "There's No Planet B". One protester held a sign reading "You'll Miss The Rains Down in Africa". Experts say Africa, with its population of more than a billion people, is expected to be hardest hit by global warming even though it contributes least to the greenhouse gas emissions that cause it.

In Helsinki, police said about 3,000 students gathered in front of Finland's parliament sporting placards such as "Dinosaurs thought they had time too!"

Berlin organiser Carla Reemtsma, a 20-year-old university student, said social media had been key in reaching people directly to co-ordinate the massive protests in so many different locations, noting that that she was in 50 WhatsApp groups and fielding 30,000 messages a day.

"It's really important that people are getting together all over the world, because it's affecting us all," she said.

Thousands of pupils from schools, colleges and universities across the UK walked out today in the second major strike against climate change this year (Image: Chris J Ratcliffe/Getty Images)

Some politicians have criticised the students, suggesting they should be spending their time in school, not on the streets.

"One can't expect children and young people to see all of the global connections, what's technically reasonable and economically possible," said the head of Germany's pro-business Free Democratic Party, Christian Lindner. "That's a matter for professionals."

But scientists have backed the protests, with thousands signing petitions in support of the students in Britain, Finland and Germany.

"We are the professionals and we're saying the young generation is right," said Volker Quaschning, a professor of engineering at Berlin's University of Applied Sciences.

Schoolchildren take part in a student climate protest in London (Image: Chris J Ratcliffe/Getty Images)

"We should be incredibly grateful and appreciative of their bravery," said Mr Quaschning, one of more than 23,000 German-speaking scientists to sign a letter of support this week. "Because in a sense, it's incredibly brave not to go to school for once."

Scientists have warned for decades that current levels of greenhouse gas emissions are unsustainable, so far with little effect.

In 2015, world leaders agreed in Paris to a goal of keeping the Earth's global temperature rise by the end of the century well below 2C, but the world is on track for an increase of 4C, which experts say would have far-reaching consequences for life on the planet.

Mrs Merkel and French President Emmanuel Macron have publicly welcomed the student protests, even as their policies have been criticised as too limited by environmental activists.

In France, activist groups launched legal action against authorities this week for failing to do enough to fight climate change, citing a similar successful effort in the Netherlands.

In Germany, environmental groups and experts have attacked government plans to continue using coal and natural gas for decades to come.

Activists say that countries like Germany should fully "decarbonise" by 2040, giving less-advanced nations a bit more time to wean themselves off fossil fuels while still meeting the Paris goal globally.